An Agreement
by Eye Greater Than Three
Summary: AU. Sirius Black and James Potter were less than pleased to see their siblings strolling through Hogsmeade. Regulus/Hippocampus (female!Harry).


**Fandom(s):** Harry Potter  
><strong>Pairing(s):<strong> Regulus Black/Hippocampus Potter, one-sided Sirius Black/Hippocampus Potter, and mentions of James Potter/Lily Evans  
><strong>Setting:<strong> Marauder's Era  
><strong>Warning(s):<strong> female!Harry (Hippocampus); alternate universe; het; and Hippocampus is James' younger sister

**Summary:** Sirius Black and James Potter were less than pleased to see their siblings strolling through Hogsmeade. Regulus/Hippocampus.

* * *

><p><strong>An Agreement<strong>

Hippocampus Potter blushed as soon as the words left Regulus Black's mouth. She peered at the Black Heir through her long eyelashes, noting the light pink color of Regulus' cheeks. He smiled and nodded, not trusting his voice. Regulus softly smiled back.

Taking a step forward, Regulus held his hand out with a hopeful look on his face. Hippocampus laced her fingers with Regulus' as the Slytherin pulled her through the streets of Hogsmeade. The pair stopped when they reached the main street. Just outside of Honeydukes was a familiar group of sixth years. James Potter and Sirius Black were laughing at something while Remus Lupin smiled around his chocolate bar and Peter Pettigrew laughed with his friends, confusion on his face.

Sharing a look, Regulus squeezed Hippocampus' hand before dropping it. Hippocampus watched him with confusion as she narrowed her green eyes. Regulus smiled softly at her as he wrapped his left arm around her shoulders.

Taking a deep breath, the pair slowly began to make their way back up to the castle. They both knew they were unlikely to be able to sneak by their older brothers, but it was worth a chance. When it came to Hippocampus, James and Sirius had a sixth sense. It was something that drove Regulus crazy for two reasons. One, he hardly had anytime to spend with Hippocampus by himself, and two, he was jealous because Sirius care more about his best friend's younger sibling than his own. It wouldn't surprise Regulus if Sirius had feelings for Hippocampus.

As they passed the group of four Gryffindors, James and Sirius stopped laughing and turned to look at Hippocampus. James' eyes narrowed when he saw Regulus' arm around his younger sister while Sirius gazed at the pair with something in his eyes.

"Hippocampus," James said, his voice hardened, "just _what_ do you think you're doing?" He eyed Regulus through narrowed eyes and a sneer crossing his features. Regulus was one of James' least favorite Slytherins. Hippocampus didn't know why since he was Sirius' younger brother.

"I'm on a date," Hippocampus replied, glaring at her older brother. James might be two years older and the Potter heir, but Hippocampus was their mother's favorite and their father's little girl. "And it's none of your business."

"You're too young to date," Sirius declared, glaring at his younger brother.

"You and James started when you were thirteen," Regulus pointed out as he tightened his grip around Hippocampus' shoulders. "We're fourteen."

"I'll tell dad," James threatened, smirking at his little sister. Charlus Potter wasn't likely to let his little girl date until she was out of Hogwarts, and even then it wouldn't be a Slytherin. James chose to ignore the fact that their mom was a Slytherin.

"There's nothing he – or mom – can do about it," Hippocampus stated. She glanced at Regulus, and the older boy nodded.

James' eyes narrowed while Sirius scowled before glaring at his younger brother. Hippocampus was _his_, even if she didn't know it yet. Sirius and James had had it planned. James would marry Lily, and they would have three children, two boys and a girl: Evan James, Sirius Charlie, and Rose Dorothy. Sirius would marry Hippocampus, and they would have four children, two of each: James Sirius, Leo Albus, Phoenix Lily, and Luna Godric.

"My parents invoked _oculus pro oculus_," Regulus explained.

James paled and Sirius mouth fell open.

"T-they wouldn't do that?" Sirius stated. If Orion and Walburga Black did that, then that would ruin his and James' plans.

_Oculus pro oculus_ was an old pure-blood agreement, of sorts. It was neither dark nor light magic, but it was binding magic. In the case of the Blacks', they used when Sirius ran away to the Potters' and they took him in. They used _oculus pro oculus_ to trade their eldest child for the Potters' youngest child. They chose Hippocampus over James since Hippocampus was the first female Potter born in over a century, and it was a bonus that she was a Ravenclaw at the top of her class. At some point, Hippocampus would become a member of the Black household like Sirius was a member of the Potter household. Regulus' parents had agreed to Regulus marry Hippocampus when he asked.

"They did," Hippocampus stated with a glare at her brother's best friend. She pushed up her robe sleeve, allowing James and Sirius to see the Egyptian eye tattoo on her inner right wrist.

Both James' and Sirius' eyes widened.

Regulus' hand tightened around Hippocampus' shoulder. Without a word, he guided her away and resumed their return to the castle. When they reached school grounds, Regulus dropped his arm from around her shoulders, and turned his body so they were facing each other.

Hippocampus stared at Regulus with a few tears in her green eyes.

Without a word, Regulus' left hand reached up to cup her cheek. "I meant what I said," he told her softly. "This doesn't change anything. I would marry you with or without this."

Hippocampus smiled and leaned into Regulus' touch. "I know," she whispered, blinking back a few tears.

Regulus stroked his thumb across her cheek. He had like Hippocampus since they met in Diagon Alley when they were nine. He had admire her from a distance. James and Sirius were protective of her, and _everyone_ knew Sirius liked her.

He leaned and pressed a kiss to her left cheek before resting his forehead against hers.

Hippocampus smiled softly, leaning into his embrace.

The pair exchanged a quick kiss before resuming their walk into the castle, hand in hand.

**...**

For Regulus and Hippocampus, the years started to spend by. They went from fourth year to fifth year. Their O.W.L.s year passed in a flurry of studying, homework, and sneaking kisses in the back of the library. Both of them passed all of their O.W.L.s with EEs and Os. Regulus ignored his housemates as they gossiped about the Dark Lord. He thought about joining _before_ he had Hippocampus, but she was far more important. While his mother didn't support this decision, his father did. Orion started training Regulus as the Black Heir, officially disowning Sirius.

Sixth year came and went, and before they knew it was seventh year. Regulus was Captain and Seeker of the Slytherin Quidditch Team while Hippocampus was Head Girl. The year went by quickly. Hippocampus graduated at the top of their class with Regulus a few places behind her.

These same years worked out differently for James and Sirius. In their seventh year, James started dating Lily. They were engaged over the Winter Holidays, and married the summer after graduation. James entered the Auror Academy, and graduated near the top of his class while Lily furthered her studies in Charms and Potions.

At the same time, Sirius dated around and developed his reputation as a playboy. He graduated Hogwarts in the top half of his class, and entered the Auror Academy with James before they joined the Department of Magical Law Enforcement.

**...**

Regulus stepped out of the fireplace and wordless waved his wand, dusting any floo powder off. Graduation was three days ago. Regulus had spent the last few days with his family, mainly his father. Orion wanted Regulus to join him in the next Wizengamot session. With a suitable heir in-training, Orion was looking into an early retirement. Of course, this idea of an early retirement had nothing to do with presenting his son as Lord Black before any of the other "Dark" Houses – particularly the Malfoys.

Quickly, Regulus exited the floo room of Potter Manor. He headed towards the informal ballroom, where he knew Charlus and Dorea Potter would be hosting an informal gathering to celebrate Hippocampus' graduation.


End file.
